As one example of the techniques of the type described above, there is conventionally known a ball-valve type double eccentric valve disclosed in Patent Document 1 listed below. This double eccentric valve is shown in sectional views of FIGS. 41 and 42 (corresponding to FIGS. 2(a) and (b) in Patent Document 1). This double eccentric valve is provided with a housing 52 having a passage 51 for fluid, a valve seat 53 including a valve hole 53a and an annular seat surface 53b formed on the edge of the valve hole 53a, a valve element 54 having a circular disc shape and an annular sealing surface 54a on its outer periphery corresponding, or conformable, to the seat surface 53b, and a rotary shaft 55 to rotate the valve element 54. Herein, the axis L1 of the rotary shaft 55 extends in parallel to a diametrical direction of the valve element 54 and the valve hole 53a and also is positioned eccentrically from the center of the valve hole 53a in a radial direction of the valve hole 53a. Further, the sealing surface 54a is placed eccentrically toward an extending direction of an axis L2 of the valve element 54 from an axis L1 of the rotary shaft 55. Furthermore, the valve element 54 is configured to rotate about the axis L1 of the rotary shaft 55 from a fully closed state where the sealing surface 54a is in surface contact with the seat surface 53b to a fully open state where the sealing surface 54a is furthest away from the seat surface 53b. In this double eccentric valve, the valve seat 53 is provided with an elastic member (an upstream-side cylindrical seal member 56, a downstream-side cylindrical seal member 57, and a surface pressure generating means 58) to hold the seat surface 53b of the valve seat 53 in pressure contact with the sealing surface 54a of the valve element 54 during full close to enhance sealing performance in the fully closed state. While fluid pressure acts on the valve element 54, the elastic member presses the valve seat 53 against the valve element 54, thereby blocking up a gap or clearance between the valve element 54 and the valve seat 53. FIGS. 41 and 42 illustrate a central cross section of the passage 51, the valve seat 53, the valve element 54, and the rotary shaft 55, taken along a line passing through the centers of the passage 51 and the valve seat 53 and extending in a direction perpendicular to the axis L1 of the rotary shaft 55.